Coextruded thermoplastic stretch-wrap

ABSTRACT

This present invention relates to novel coextruded thermoplastic film and the employment of such multi-layer film as stretch-wrap material for packaging of goods, including relatively large palletized loads of material. More specifically, such coextruded stretch-wrap films comprise three-layer laminations having a relatively thin skin layer and a relatively thicker core layer. Suitable skin layers include highly-branched low-density polyethylene, and suitable core layers include linear low-density polyethylene co-polymers, such as ethylene co-polymerized with a minor amount of at least one C 4  to C 10  alpha-olefin, such as octene-1 and 4-methyl-pentene-1, and butene-1.

BACKGROUND OF THE INVENTION

This application is a continuation application of now abandonedapplication Ser. No. 187,678, filed Sept. 16, 1980, which in turn is acontinuation-in-part application of now abandoned Ser. No. 942,715,filed Sept. 15, 1978.

The present invention relates to thermoplastic film structures, inparticular plastic film structures which have been formed utilizingcoextrusion techniques. The laminate comprises a core of a linearlow-density polyethylene having exterior skin layers of low-densitypolyethylene, i.e., conventional polyethylene prepared utilizing theprior art free-radical high pressure polymerization process.

The use of thermoplastic stretch-wrap for the overwrap packaging ofgoods, in particular the unitizing of pallet loads, is a currentlycommercially developing end use application for thermoplastic films,including polyethylene. There are a variety of overwrapping techniqueswhich are employed utilizing such stretch-wrap films, including locatingthe pallet load to be wrapped atop a rotating platform. As polyethylenefilm is laid on about the girth of the pallet load, the pallet load isrotated on its platform. The polyethylene stretch-wrap is applied from acontinuous roll thereof. Braking tension is applied to the continuousroll of film so that the film is being continuously stretched by therotating pallet load. Usually the stretch-wrap film located adjacent tothe rotating pallet load is vertically positioned and the rotatingplatform or turntable may be operated at speeds ranging from about 5 upto about 50 revolutions per minute. At the completion of the overwrapoperation the turntable is stopped completely while the film is cut andattached to the previous layer of film employing tack sealing, tape,spray adhesives or a cling-modified film whereby overlapping layers ofthe stretch-wrap have a pronounced tendency to cling together at theirinterface. Depending upon the width of the stretch film roll, the loadbeing overwrapped may be shrouded in the film while the verticallypositioned film roll remains fixed in a vertical position, or thevertically positioned film roll (e.g., in the case of relatively narrowfilm widths and relatively wider pallet loads) may be arranged to movein a vertical direction as the load is being overwrapped whereby aspiral wrapping effect is achieved on the packaged goods.

Stretch films employed in the prior art have included film materialssuch as polyethylene, polyvinyl chloride and ethylene vinyl acetate.

With respect of the ethylene vinyl acetate type of stretch filmproducts, the prior art has employed a percentage by weight of vinylacetate in the co-polymers of about 2% up to about 15% and preferablyfrom about 4% up to about 12% by weight for stretch film applications.

Physical properties which are particularly significant for thesuccessful use of thermoplastic films in stretch-wrap applicationsinclude their puncture resistance, their elongation characteristics,their toughness, and their resistance to tearing while under tension. Inparticular, the latter physical characteristics of such film, i.e.,their resistance to tearing and their resistance to puncture, areparticularly significant. In general tensile toughness is measured as anarea under a stress-strain curve for a thermoplastic film, or it may beconsidered as the tensile energy absorbed, expressed in units offt.-lbs./in.cu. to elongate a film to break under tensile load. In turn,this toughness characteristic is a function of the capacity of suchfilms to elongate. The process of stretching the film decreases thatcapacity. Accordingly, the stretch-wrap process will decrease thetoughness of the film while it is in its stretched condition as anoverwrap as compared to unstretched counterparts, including suchmaterials as shrink-wrap. Generally this loss of toughness isproportional to the amount of stretch imparted to the film as it isoverwrapping a load of goods.

As hereinabove indicated, the resistance to tear characteristic of suchfilms will be obviously an important physical characteristic forstretch-wrap applications since if the edge of the stretch film roll isnicked, abraded or in any way weakened before stretching or during thestretching operation, premature tearing of the film will usually occurduring wrapping or subsequent handling of the load of goods.

In practice, one commonly accepted technique for properly tensioning afilm around a load such as a pelletized load is to adjust the brakingforce on the roll unit a significant amount of neck-in (i.e., film widthreduction) occurs. Alternatively film may be tensioned until aninitiated tear results in unrestricted propagation of the tear acrossthe film width.

It is an object of the present invention to provide a stretch filmmaterial which, unlike currently commercially available stretch films,is a laminar structure comprising at least two and preferably three filmlayers. The prior art stretch film materials hereinabove referred to,such as polyvinyl chloride, ethylene vinyl acetate co-polymer andlow-density polyethylene, have been found to offer reduced resistance totear in both the film's machine direction and transverse direction aswell as reduced toughness and elongation characteristics in contrast tothe laminar film compositions of the present invention.

SUMMARY OF THE PRESENT INVENTION

In accordance with the present invention, a stretch-wrap material isprovided which comprises a primary layer of a linear low-densitypolyethylene film, which primary layer has a coextruded layer on atleast one side thereof comprising a highly branched low-densitypolyethylene fabricated utilizing a high pressure free-radicalpolymerization process. The preferred linear low-density polyethylenesconsist essentially of ethylene co-polymerized with minor amounts ofanother olefinic hydrocarbon having four to ten carbon atoms, includingsuch materials as octene-1, 4-methyl-pentene-1, hexene-1, butene-1 anddecene-1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As hereinabove discussed, the present invention comprises the formationof a laminar stretch-wrap thermoplastic film by initially preparing thecoextruded stretch-wrap product utilizing conventional coextrusiontechniques. The material construction of the laminate prepared inaccordance with the following example comprises a core layer of linearlow-density polyethylene, the linear low-density material comprisingethylene which has been copolymerized with a minor amount of octene-1.Linear low-density ethylene co-polymers are commercially availablematerials and are manufactured by low pressure processes employingstereospecific catalysts. These materials usually contain 1 to 10 wt.%of C₄ to C₈ α-olefin hydrocarbon copolymerized with ethylene, insufficient amount to give 5 to 15 branches per thousand carbon atoms inthe linear polymer. Manufacturing processes for linear low densitypolyethylenes are disclosed in U.S. Pat. Nos. 4,076,698, and 4,205,021.

The exterior skin layers are fabricated from highly-branched low-densitypolyethylene resin produced by the high pressure process. The highpressure low-density polyethylene skin layer provides the requisitecling and gloss properties necessary for stretch film applications. Thelinear low-density polyethylene which contains the core layer impartsthe desired tear and puncture resistance as well as the toughness whichis required of a film in such a new use application.

In the following Table A the physical properties of the low-densitypolyethylene and the linear low-density polyethylene resins which wereemployed to fabricate the films identified as X-1, X-2 and X-3 reportedin Table 2 are set forth below:

                  TABLE A                                                         ______________________________________                                                          LLDPE    LLDPE    LLDPE                                                       Core     Core     Core                                      LDPE-Skins (homopolymers)                                                                       X-1      X-2      X-3                                       ______________________________________                                        Density (g/cc)    0.9202   0.9228   0.9186                                    Melt Index        2.3      2.1      2.4                                       Molecular Weight                                                              Wgt. Avg.         99,100   96,300   --                                        No. Avg.          13,800   20,200   --                                        ______________________________________                                    

Also reported in Table 2 are the physical properties of a currentlyavailable LDPE laminar stretch film comprising two layers of highpressure (low-density) polyethylene. One layer had a density of 0.925and a melt index of 1.4. The second layer had a density of 0.918 and amelt index of 7.0.

EXAMPLE 1

Linear low-density polyethylene as hereinabove defined was fed into thefeed hopper of a conventional rotating screw extruder. The extruderscrew employed has a 6" diameter and a length to diameter ratio of about24:1. The satellite extruder which was employed for the extrusion of thehereinabove low-density polyethylene material comprised a conventionalextruder having an extruder screw with a 3.5" diameter and a length todiameter ratio of about 24:1. Molten resin from the satellite extruderwas fed into the cast film die affixed to the end of the core extruder,through an adapter specifically designed to join the polymer stream fromthe satellite extruder to the molten polymer core stream so that itcovers and encompasses the molten surfaces of the core layer. A morecomplete description of this prior art process may be found in U.S. Pat.No. 3,748,962, the disclosure of which is incorporated herein byreference. The specific line conditions employed in the present exampleare set forth in the following table:

                  TABLE 1                                                         ______________________________________                                                        SKIN RESIN                                                                    LDPE   LDPE                                                                   CORE RESIN                                                                    LDPE   Ethylene-octene-1                                      ______________________________________                                        Melt Temperature                                                              Skin (°F.) 520      520                                                Core (°F.) 565      575                                                Line Speed (FPM)  715      635                                                Chill Roll Temperature (°F.)                                                             75       75                                                 Extruder Screw Speed (RPM)                                                    Satellite         65       65                                                 Main              110      85                                                 Skin Percentage % by wgt.                                                                       15       15                                                 Gauge of Laminate (mils)                                                                        1.0      1.0                                                % Octene-1 by Wgt.         12%                                                ______________________________________                                    

Although the present example describes a cast film process for themanufacture of the present stretch film products, it will be understoodthat other conventional thermoplastic film forming techniques may beemployed, such as the commonly employed tubular extrusion processutilizing an entrapped air bubble to expand the extruded film tube. Thefilm produced in accordance with the present example comprises a linearlow-density polyethylene core consisting of about 85% by weight of theover-all laminar product, while the exterior high pressure low-densitypolyethylene skins contributed about 71/2% by weight per side. The gaugeof the composite laminar structure ranged from about 0.8 up to about 1.0mil.

The physical properties of film produced in accordance with Example 1and identified in the following Table 2 as X-1, X-2, and X-3 are setforth below. Additionally, in Table 2, for comparative purposes, thephysical properties of currently commercially available stretch-wrapmaterials, including polyvinyl chloride, ethylene vinyl acetate, and atwo layer low-density polyethylene are set forth.

                                      TABLE 2                                     __________________________________________________________________________    Ethylene-α-olefin Coextrusion                                                            X-1 X-2 X-3                                                                              PVC                                                                              EVA LDPE                                       __________________________________________________________________________    Caliper (mils)   1.0 1.1 0.93                                                                             0.8                                                                              1.0 1.0                                        ASTM D-882                                                                     Ultimate Tensile PSI                                                                      MD  4200                                                                              5400                                                                              6542                                                                             4900                                                                             5400                                                                              3600                                                    TD  3300                                                                              3700                                                                              4459                                                                             4000                                                                             4500                                                                              2300                                        Yield (PSI) MD  1900                                                                              1300                                                                              958                                                                              1600                                                                             900 1300                                                    TD  1100                                                                              1300                                                                              963                                                                              1000                                                                             800 1300                                        Elongation (%)                                                                            MD  500 650 597                                                                              300                                                                              450 500                                                     TD  900 900 907                                                                              300                                                                              600 700                                        ASTM D-1922                                                                    Elmendorf Tear - g/mil                                                                    MD  150 90  130                                                                              80 35  150                                                     TD  700 960 798                                                                              120                                                                              75  350                                        ASTM D-882                                                                     Toughness (Ft. lbs/in.sup.3)                                                              MD  1100                                                                              1500                                                                              1670                                                                             800                                                                              1300                                                                              1050                                       Puncture                                                                       Instron Penetration                                                                       Lbs.                                                                              10  11  9.5                                                                              12 15  8                                          Energy                                                                         Rupture     In.-Lbs                                                                           36  37  39.9                                                                             19 44  12                                         Penetration                                                                    Instron Probe                                                                             In. 5   5   6.2                                                                              3  5   3                                          Cling Index      --  1.0 2.4                                                                              2.3                                                                              3.5 2.2                                        ASTM D-2457                                                                    (Gloss (% at 45°)                                                                      87  85  89.9                                                                             87 74  89                                         ASTM D-1003                                                                    Haze (%)        1.5 2.2 0.8                                                                              1  2   1                                          Density (g/cc)   0.9151                                                                            0.9174                                                                            -- 1.23                                                                             0.9313                                                                            0.9185                                     __________________________________________________________________________

It has been found that the types of high pressure, low-density skinresins employed in the present invention may vary in physicalcharacteristics. Preferred skin resins however include those withdensities of from about 0.917 up to about 0.922 and melt indices of fromabout 4 up to about 8. The preferred linear low density polyethyleneco-polymer core resins include those with densities of from about 0.916up to about 0.925 with melt indices of from about 1.0 up to about 6.0.The thicknesses of the skin layers may vary widely, however preferredthicknesses include those from about 5% up to about 40% based upon theoverall thickness of the laminate.

It is to be understood that the foregoing description is merelyillustrative of preferred embodiments of the invention, of which manyvariations may be made by those skilled in the art within the scope ofthe following claims without departing from the spirit thereof.

What is claimed is:
 1. A thermoplastic film adapted for use as a stretchfilm wrap comprising a coextruded three-layer thermoplastic film havinga core layer comprising a linear low-density polyethylene, said linearlow-density polyethylene consisting essentially of ethyleneco-polymerized with a minor amount of at least one alpha-olefin havingfour to ten carbon atoms and exterior skin layers comprisinghighly-branched low-density polyethylene.
 2. A film according to claim 1wherein said core layer consists essentially of linear low-densitycopolymer of ethylene with a minor amount of octene-1, having a densityof about 0.916 to 0.925 and a melt index of about 1.0 to 6.0.
 3. A filmaccording to claim 1 wherein said linear low-density polyethylenecontains about 1 to 10 weight % total of butene-1, 4-methyl pentene-1,octene-1 or mixtures thereof.
 4. A film according to claim 1 whereinsaid core layer has a density of about 0.916 to about 0.925 and a meltindex of about 1.0 to about 6.0 and said skin layers have a density ofabout 0.917 to about 0.922 and a melt index of about 4 to about 8.